Steam Engines Machinery's Reference Series, Number 70 by Anonymous

Author:Anonymous [Anonymous]
Language: eng
Format: epub
Published: 2010-12-19T05:00:00+00:00

Steam Consumption and Ratio of Expansion

The steam consumption is commonly called the water rate, and is expressed in pounds of dry steam required per indicated horsepower per hour. This quantity varies widely in different types of engines, and also in engines of the same kind working under different conditions. The water rate depends upon the “cylinder losses,” which are due principally to condensation, although the effects of clearance, radiation from cylinder and steam chest, and leakage around valves and piston, form a part of the total loss. Table IV gives the average water rate of different types of engines working at full load.

The most economical ratio of expansion depends largely upon the type of the engine. In the case of simple engines, the ratio is limited to 4 or 5 on account of excessive cylinder condensation in case of larger ratios. This limits the initial pressure to an average of about 90 pounds for engines of this type. In the case of compound engines, a ratio of from 8 to 10 is commonly employed to advantage, while with triple-expansion engines, ratios of 12 to 15 are found to give good results.

The thermal efficiency of an engine is the ratio of the heat transformed into work to the total heat supplied to the engine. In order to determine this, the absolute temperature of the steam at admission and exhaust pressures must be known. These pressures can be measured by a gage, and the corresponding temperatures taken from a steam table, or better, the temperatures can be measured direct by a thermometer. The absolute temperature is obtained by adding 461 to the reading in degrees Fahrenheit (F.). The formula for thermal efficiency is:

T1 - T2

———

T1

in which

T1 = absolute temperature of steam at initial pressure.

T2 = absolute temperature of steam at exhaust pressure.

Example:—The temperature of the steam admitted to the cylinder of an engine is 340 degrees F., and that of the exhaust steam 220 degrees F. What is the thermal efficiency of the engine?

(340 + 461) - (220 + 461)

Thermal efficiency = ——————————— = 0.15

340 + 461

The mechanical efficiency is the ratio of the delivered or brake horsepower to the indicated horsepower, and is represented by the equation:

B. H. P.

Mechanical efficiency = ———

I. H. P.

in which B. H. P. = brake horsepower,

I. H. P. = indicated horsepower.

All engines are designed to give the best economy at a certain developed indicated horsepower called full load. There must, of course, be more or less fluctuation in the load under practical working conditions, especially in certain cases, such as electric railway and rolling mill work. The losses, however, within a certain range on either side of the normal load, are not great in a well designed engine. The effect of increasing the load is to raise the initial pressure or lengthen the cut-off, depending upon the type of governor. This, in turn, raises the terminal pressure at the end of expansion, and allows the exhaust to escape at a higher temperature than before, thus lowering the thermal efficiency.

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